If the .NET application is running in a different user account (for example, a Windows service), edit the machine.config file:

<!-- The following section is to force use of Fiddler for all applications, including those running in service accounts --> <system.net>
<defaultProxy>
<proxy autoDetect="false" bypassonlocal="false" proxyaddress="http://127.0.0.1:8888" usesystemdefault="false" />
</defaultProxy>
</system.net>

Note: Important: Regardless of other settings, .NET will always bypass the Fiddler proxy for URLs containing localhost. So, rather than using localhost, change your code to refer to the machine name. For instance: